High-alumina ceramic composition

ABSTRACT

A high-alumina ceramic composition is described, having a low dielectric loss in the extremely high frequency range, said composition consisting essentially of Al 2  O 3 , SiO 2 , and MgO, with the Al 2  O 3  content being in the range of from 99.7 to 99.9% and the SiO 2  /MgO weight ratio being in the range of from 1.0/1 to 0.2/1.

BACKGROUND OF THE INVENTION

The present invention relates to a high-alumina ceramic compositionhaving a small dielectric loss in the high frequency range.

Conventional waveguides used with electronic circuits designed tooperate with millimeter waves in the high frequency range (≧30 GHz) aretypically made of metals. Recently, titanates such as barium titanate,strontium titanate, and calcium titanate, as well as alumina ceramicshave been considered as promising alternatives to metals as the materialfor waveguides operating with millimeter waves. However, titanates havea dielectric loss tangent (tan δ) of the order of 10⁻⁴ and are too highin dielectric loss to make the titanates suitable for operation withmillimeter waves. Alumina ceramics having a satisfactory low tan δ arevery difficult to produce on a commercial scale, and hence are notsuitable for use in practical applications.

SUMMARY OF THE INVENTION

The present invention provides a ceramic composition consistingessentially of Al₂ O₃, SiO₂, and MgO, with the Al₂ O₃ content being inthe range of from 99.7 to 99.9% and the SiO₂ /MgO weight ratio being inthe range of 1.0/1 to 0.2/1. By controlling the SiO₂ /MgO ratio withinthis specified range, the ceramic composition of the present inventionhas a dielectric loss tangent (tan δ) of the order of as low as 10⁻⁵,even if the alumina content is reduced to a content which is suitablefor use in practical application, that is, 99.7%.

DETAILED DESCRIPTION OF THE INVENTION

As the result of extensive investigations, the present inventors havefound that alumina ceramics have a satisfactorily low tan δ, of theorder of about 10⁻⁵ if their alumina content is 99.9% (by weight) orhigher. However, alumina ceramics with an Al₂ O₃ content of 99.9% ormore are very difficult to produce on a commercial scale, and hence arenot suitable for use in practical applications, as hereinafterdescribed.

The reason why the alumina ceramics that can be used as the material forwaveguides designed to operate with millimeter waves must have analumina content of at least 99.9% is as follows. If sintered aluminacomprises large crystal grains, the resultant increase in anisotropycauses an increased dielectric loss. In order to avoid this problem, MgOand SiO₂ have been added as a crystal growth restrainer and a sinteringaid, respectively. However, these additives produce crystals other thanAl₂ O₃ at the grain boundaries of the alumina crystals; MgO forms spinel(Al₂ O₃.MgO) and SiO₂ forms mullite (3Al₂ O₃.2SiO₂). These undesiredcrystals also cause an increased tan δ. Therefore, in order to avoid theformation of spinel and mullite due to the reaction of MgO and SiO₂ withAl₂ O₃, the addition of these two compounds should be minimized.

When the alumina ceramics which have an alumina content of 99.9% orhigher are produced, high purity starting alumina (for example, 99.995%pure alumina) must be used. However, when the alumina ceramics of thepresent invention are produced, the starting alumina may have a purityof 99.7% or higher. The starting alumina is conventionally produced byvarious processes such as Bayer's process, a process comprising athermal decomposition of ammonium carbonate, a process comprising ahydrolysis of alcoholate or the like. An inexpensive starting aluminahaving a purity of less than 99.9% is almost produced by Bayer'sprocess. The impurities thereof mainly contain SiO₂ which is known assintering aid of alumina. Therefore, the alumina ceramics of the presentinvention are suitable for production on a commercial scale, because theinexpensive starting alumina produced by the conventional processes asdescribed above can be used.

The reason why the SiO₂ /MgO weight ratio was controlled within therange of 1.0/1 to 0.2/1 is hereinafter explained. When SiO₂ was added inan excess amount beyond the specified ratio range, an effect of MgO wasdecreased and an abnormal grain growth of alumina was encouraged to forman alumina ceramics containing a number of pores and causing anincreased tan δ. When MgO was added in an excess amount beyond thespecified ratio range, dense alumina ceramics having uniform aluminacrystal grains could be formed. However, these crystals also caused anincreased tan δ. This is believed to be ascribable to crystallographicfactors (such as a lattice defect or a lattice distortion).

The present invention is hereunder described in greater detail byworking example which is given here for illustrative purposes only andby no means intended to limit the scope of the invention.

EXAMPLE

High-purity (99.995%) alumina (UPA, product of Mitsubishi KeikinzokuK.K.), anhydrous silicic acid (SiO₂, guaranteed reagent) and magnesiumcarbonate (MgCO₃, guaranteed reagent) were mixed in the weight ratiosshown in the following Table. Each mixture had a total weight of 1,000g. The amount of MgCO₃ is shown in the Table in terms of MgO. To eachpowdery mixture was added deionized water (1,000 ml), polyvinyl alcohol(10 g) and polyethylene glycol (5 g). Each mixture was ground in apolyethylene ball mill (inner capacity: 2,000 ml) with 99.995% purealumina balls for 15 hours. Each of the resulting slurries wasspray-dried through an atomizer (110 mm diameter; Centrifugal SprayDrier; product of Yanagi-Seisakusho Co., Ltd.) at a gas temperature of170° C. and 7,200 rpm.

The grains obtained had an average size of 50 μm and were rubber-pressedat 1,500 kg/cm² (Vertical Single Cylinder-Type High Pressure RubberPressing Apparatus; product of Matsuura-Koatsukikai Co., Ltd.) to formcylinders (4 mm diameter×100 mm length). After drying, the cylinderswere baked in an electric furnace in an oxidizing atmosphere at fromabout 1,540° to 1,560° C. for 1 hour to obtain cylindrical sinteredproducts (3.3 mm diameter×83.3 mm length).

Each of the sintered products was ground with a centerless outsidediameter grinder and a diamond cutter to prepare test pieces measuring 3mm diameter×5.0 mm length. The test pieces were washed first with waterand then with chlorothene and roasted in an electric furnace at 1,200°C. for 1 hour. The dielectric loss tangent (tan δ) of each test piecewas measured using the blow identified apparatus under the notedconditions, and the results are shown in the Table.

Apparatus: Model 8408B, product of Yokogawa-Hewlett-Packard Co., Ltd.

Frequency: 7.4 GHz

Temperature: 20° C.

                                      TABLE                                       __________________________________________________________________________                         Sintering         Average                                Sam-                 Temper-                                                                            Dielectric                                                                            Di-  Grain                                                                              Specific                          ple                                                                              Composition (wt %)                                                                              ature                                                                              Loss Tangent                                                                          electric                                                                           Size Gravity                           No.                                                                              Al.sub.2 O.sub.3                                                                  SiO.sub.2                                                                         MgO SiO.sub.2 /MgO                                                                      (°C.)                                                                       (tan δ × 10.sup.-5)                                                       Constant                                                                           (μm)                                                                            (g/cm.sup.3)                                                                       Remarks                      __________________________________________________________________________    1  99.7                                                                              0.30                                                                              0   ∞                                                                             1,550                                                                              25.6    9.58 2.2  3.91 SiO.sub.2 /MgO outside                                                        the scope                                                                     of the present                                                                invention                    2  "   0.20                                                                              0.10                                                                              2/1   "    10.2    9.68 1.9  3.93 SiO.sub.2 /MgO outside                                                        the scope                                                                     of the present                                                                invention                    3  "   0.15                                                                              0.15                                                                              1/1   "    6.1     9.70 1.8  3.94 Samples of the present                                                        invention                    4  "   0.10                                                                              0.20                                                                              0.5/1 "    4.8     9.72 1.7  "    Samples of the present                                                        invention                    5  "   0.05                                                                              0.25                                                                              0.2/1 "    6.7     9.71 1.5  "    Samples of the present                                                        invention                    6  "   0   0.30                                                                              0/1   "    10.1    9.70 1.4   3.935                                                                             SiO.sub.2 /MgO outside                                                        the scope                                                                     of the present                                                                invention                    7  99.9                                                                              0.10                                                                              0   ∞                                                                             1,560                                                                              13.8    9.60 2.8  3.92 SiO.sub.2 /MgO outside                                                        the scope                                                                     of the present                                                                invention                    8  "   0.06                                                                              0.04                                                                              1.5/1 "    9.6     9.70 2.1  3.94 SiO.sub.2 /MgO outside                                                        the scope                                                                     of the present                                                                invention                    9  "   0.05                                                                              0.05                                                                              1/1   "    4.2     9.75 2.0  3.95 Samples of the present                                                        invention                    10 "   0.017                                                                             0.083                                                                             0.205/1                                                                             "    4.8     9.76 1.7  "    Samples of the present                                                        invention                    11 "   0   0.10                                                                              0/1   "    9.5     9.74 1.6  3.94 SiO.sub.2 /MgO outside                                                        the scope                                                                     of the present                                                                invention                    12 99.6                                                                              0.10                                                                              0.20                                                                              0.5/1 1,540                                                                              11.1    9.61 1.8  3.93 Al.sub.2 O.sub.3 outside                                                      the scope of                                                                  the present                  __________________________________________________________________________                                                     invention                

As is clear from the Table, Sample Nos. 3, 4 and 5 with 99.7% Al₂ O₃ andhaving SiO₂ /MgO weight ratios in the range of from 1/1 to 0.2/1, aswell as Sample Nos. 9 and 10 with 99.9% Al₂ O₃ and SiO₂ /MgO weightratios of from 1/1 to 0.2/1 had low dielectric loss tangents (tan δ),viz., in the range of from 4.2 to 6.7×10⁻⁵. However, Sample Nos. 1, 2and 6 with 99.7% Al₂ O₃ having SiO₂ /MgO weight ratios which wereoutside the range of 1/1 to 2/1, as well as Sample Nos. 7, 8 and 11 with99.9% Al₂ O₃ and SiO₂ /MgO weight ratios which were also outside therange of 1/1 to 0.2/1 had high dielectric loss tangents in the range offrom 9.5 to 25.6×10⁻⁵. Sample No. 12 which had an optimum SiO₂ /MgOratio of 0.5 (equal to that of Sample No. 4) but whose Al₂ O₃ contentwas less than 99.7%, did not have a desirably low dielectric losstangent (11.1×10⁻⁵).

As shown above, the high-alumina ceramic of the present invention hassuch a low dielectric loss tangent that it can be satisfactorily used asa material for waveguides to be used with an electronic circuit designedto operate at frequencies of 30 GHz or higher without suffering fromhigh dielectric loss.

The starting alumina used in the production of the composition of thepresent invention should have a high purity. In the Example shown above,99.995% pure alumina was used, and for the purpose of the presentinvention, the purity of the starting alumina should preferably be atleast 99.7%. For the same reason, anhydrous silica acid and magnesiumcarbonate used as the other starting materials should have a high purityand in the Example, guaranteed reagents were used for both compounds.However, these compounds are added in very small quantities, so theyneed not necessarily be as pure as the reagent grade.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A high-alumina ceramic composition having a dielectric loss tangent of 6.7×10⁻⁵ or less, said composition consisting essentially of Al₂ O₃, SiO₂, and MgO, with the Al₂ O₃ content being in the range of from 99.7 to 99.9% and the SiO/MgO weight ratio being in the range of from 1.0/1 to 0.2/1. 